Polynitrogen compounds have been actively pursued driven by their potential as ultrahigh-performing propellants and explosives. Despite remarkable breakthroughs in the past two decades, conventional chemistry methods have fallen short of fully exploiting the potential of polynitrogen compounds. The pressure parameter, however, has recently been demonstrated to allow the synthesis of exotic and highly energetic novel polynitrogen (poly-N) entities with relevance as high energy density materials (HEDM).

In particular, the synthesis of the pentazolate anion in the (Li)⁺(N₅)^- salt at only 45 GPa showcased the proclivity of highly reducing elements to rupture the N₂ triple bond at moderate pressure and to increase the crystal’s cohesive energy through strongly ionic bonds, ultimately allowing the compound’s retrieval at ambient conditions [Laniel et al., J. Phys. Chem. Lett. 9, 1600–1604, 2018]. With a formal charge of 2+, alkaline earth elements are the next logical step to obtain poly-N species as they are expected to favor the low pressure formation of salts with an even greater energy density and an enhanced stability. Indeed, at only 15 GPa, the MgN₄ and MgN₁₀ compounds, both composed of energetic polynitrogen anions, were calculated to be stable [Xia et al., J. Phys. Chem. C 123, 16, 2019]. Despite their obvious appeal, until now nitrogen-rich alkaline earth materials have never been obtained.

Here, molecular nitrogen was compressed along with pure magnesium up to 60 GPa and laser-heated to 3000 K. Under these conditions, two new Mg-N solids were produced and characterized by X-ray diffraction as well as Raman spectroscopy measurements. Their crystalline structure was solved by single-crystal X-ray diffraction experiments and they were shown to possess the Mg₂N₄ and MgN₄ chemical compositions. The (Mg)²⁺(N₄)^2- salt is composed of 8-fold (cubic) N coordinated Mg²⁺ atoms while nitrogen forms exotic, highly energetic, anionic polythiazyl-like 1D N-N chains.

The second detected reaction product, the (Mg₂)⁴⁺(N₄)^4- salt, adopts a complex structure constituted of cis-tetrazene like tetranitrogen anions, here synthesized for the first time. Characteristic of energetic entities, the N-N covalent bonds have a bond order comprised between one and two. Remarkably, investigation of this novel salt during its decompression exhibited its metastability down to ambient conditions as it was unambiguously identified by single-crystal X-ray diffraction after the complete release of pressure. Theoretical calculations were also performed and highlight the compound’s ionicity, stability, and provide further insight into the solid’s properties.

The high pressure synthesis and retrieval at ambient conditions of this new anionic polynitrogen entity adds the unique (N₄)^4- anion to a yet very short list of poly-N entities bulk stabilized at ambient conditions. Moreover, it establishes high pressure as an efficient method not only to discover new forms of poly-N, but also to produce novel chemical species relevant at ambient conditions and provides a new building block to synthesize new high performance nitrogen-based HEDM.